This invention relates generally to the construction of pressure-dependent capacitive sensing switches.
Capacitive sensing technology has become popular for use in touch switches covering contact, non-contact and two-dimensional user interface implementations.
As in most technologies, capacitive sensing technologies have strong points like cost, easy form factor integration, and selectable or adjustable force (contact) operation. These technologies also work with various overlay materials. The technology has become popular for user interface implementation, arguably since the year 2000. Various different implementations are used (see products from Cypress, Atmel, Microchip and Azoteq) and capacitive sensors feature in several trend-setting products (e.g. the iPod® and the iPhone® from Apple).
At present however capacitive sensing is seen as lacking certain features that are promoted as being the strength areas of inductive sensing technologies.
Specifically, inductive sensing is promoted as being superior in its robustness against RF noise and general noise, and as a much better technology with regard to immunity against accidental activation (see Microchip Inductive Sensing Products). A product like the iPhone® from Apple is sophisticated enough to combat accidental activation in a user's pocket through the requirement that a slide action must be performed quite accurately to activate the product. This type of complexity is not always possible, particularly if a user interface only comprises buttons, or if an unlocking sequence would not be acceptable.
Another problem is the presence of water or other fluids and objects coming into contact with the capacitive sensing structure. This may lead to accidental proximity or touch detections.
Inductive sensing is promoted as a good solution against accidental activation as it essentially depends on a physical deflection between a surface (electrically conductive or with magnetic permeability) and an excitable coil. Since the surface is electrically conductive it can act as a shield which helps to deliver very robust performance in noisy environments.
It is an object of this invention to provide a capacitive sensing technology implementation to overcome a number of the perceived shortcomings of capacitive sensing as stated in the art, as discussed above. It is also shown that this capacitive sensing implementation matches inductive sensing on a number of its perceived strong points. As capacitive sensing comprises a simple technology, implementable at low cost, with low power consumption and offers reliable operation, this holds definite market value.